The application of geophysical electrical measurements as an oil exploration aid dates back to experiments performed by Conrad Schlumberger circa 1912 to measure variations in the resistivity of geologic formations. His legacy is the giant oil services company that bears his name.
Subsequently, these techniques were applied to mineral exploration and more recently to environmental investigations. Geophysical electrical measurements today, nearly a century after they were first made, are still focussed primarily on resistivity. Variations in this parameter yield important information in both environmental and mineral exploration applications. There are two distinct modes of operation in all cases; measurements which are made on the surface and those which are made in boreholes. In each case the simplest technique is to use two electrodes in contact with the formation and measure the potential drop when a current of known value is passed between them. A more sophisticated implementation is to use four electrodes, two for generating a current in the formation and two for measuring the potential drop across a small segment of the formation through which the current is passing. Both techniques are referred to as galvanic resistivity measurements, after Galvani, the eighteenth century Italian scientist who first investigated the electrical phenomena.
In the case of borehole galvanic resistivity measurements using four probes, one current electrode is installed at the surface, while the other current electrode and the two potential electrodes are installed in a probe that is lowered down the hole. Such measurements require good electrical contact between the electrodes in the probe and the wall of the borehole.
In several instances, particularly in hydro-geological and environmental applications, the boreholes are cased with plastic, making the use of the galvanic techniques impossible. For many years an alternate solution has made use of an inductive electromagnetic technique, commonly referred to a “EM”, involving a probe having a transmitter coil and a receiver coil. The transmitter generates an alternating magnetic field that is detected by the receiver. The technique relies on the fact that currents are induced in the surrounding formation by the alternating field, and in turn the field is modified according to the resistivity of the formation. The signal detected by the receiving coil thus reflects these modifications and enables the variations in resistivity to be recorded.
More recently a technique known as capacitive measurement has been demonstrated for resistivity measurements in geophysical applications. A capacitor is essentially a device consisting of two closely spaced parallel conducting plates, separated by an electrical insulator. The insulator can be air or some other material chosen for certain desirable properties, such as high voltage breakdown or high dielectric constant.
When the plates of a capacitor are connected to an electrical oscillator circuit, an alternating electric field is generated between them. Normally this field is confined to the region between the plates, as shown in FIG. 1A. If, however, the plates are laid on a flat surface side-by-side, as in FIG. 1B, then the electric field 10 takes the form of semi-circles arching from one plate to the other on both sides like two rainbows. The dashed lines indicate lines of electric potential 12. Conductive material that intersects the extended electric field modifies its intensity. By analogy with the EM technique described above, the modification of electric field intensity can be detected by an adjacent second capacitor (the receiver capacitor).
There have been a number of papers describing both industrial and geophysical measurement systems based on the principle of dual capacitors. Parameters measured with the capacitive technique include the resistivity and dielectric constant of the material that is intersected by the electric field.
A Russian geophysicist, V. Timofeev, designed a borehole probe incorporating resistivity measurements using transmitting and receiving capacitors. FIG. 2 shows the end-to-end cylindrical configurations used by Timofeev in his borehole probe design. Two cylinders 14 form the plates of the “transmitting” capacitor in the excitation unit, while two other cylinders 16 form the plates of the “receiving” capacitor in the measurement unit.
The borehole probe was successfully demonstrated in collaboration with the GSC in 1995 and the technique has been presented at several conferences. However, subsequent attempts to create a commercially viable borehole unit from the original prototype have been unsuccessful, although a surface version of the design was more promising, see for example Timofeev, V. M. et al; A new ground resistivity method for engineering and environmental geophysics; Proceedings of the Symposium on the Application of Geophysics to Engineering and Environmental Problems (SAGEEP); EEGS; pp. 701–715; 1994, and has since seen commercial adoption, see for example the Geometrics OmhMapper, http://www.geometrics.com.
The primary reason for the lack of commercial interest in the borehole unit is the daunting complexity of the circuitry required to produce a detectable signal from the receiving capacitor, where the signal to noise ratio is very low. The prototype probe is also bulky and difficult to operate, requiring considerable set-up time with additional specialized electronics to perform signal conditioning at the surface.
In the field of instrumental borehole measurements for mineral exploration, boreholes come in many different varieties, depending on the purpose for which they are drilled. The diameters vary from about 2″ for diamond drill holes in hard rock (very expensive to drill) to as much as 12″ for environmental holes drilled to monitor environmental parameters. Some have plastic casing, while others do not. Most holes are drilled by mining companies for exploration purposes in geologic environments where the water tables are close to the surface, indicating these hole are fluid-filled. The borehole fluid is generally water with dissolved organic salts and the resistivity of the formation in this case is primarily a function of the porosity of the rocks, which determines the amount of fluid that permeates them. Other boreholes, particularly the larger diameter boreholes, are likely to be dry. Primary constraints arise from the requirement to fit a measurement apparatus into a tube of typically two inches or less in diameter and to transmit the measurement information via a cable up two kilometers to the surface. At a depth of two kilometers, the fluid pressure of water-filled holes is 3000 psi. While the size and complexity of measurement apparatus is not particularly significant for surface measurements, in the context of borehole measurements, cumbersome and costly probes are a liability. Probes can be lost by becoming wedged in a hole or destroyed due to a pressure seal failure. These problems have been a major deterrent in the adaptation of existing surface measurement capacitive resistivity systems for borehole measurements.